Background: Optical properties of citrate capped dodecahedral gold nanoparticles have immense applications in a large variety of fields. The interband transition has a role in determining the optical behaviour of gold nanoparticles. Interband transition in citrate capped colloidal gold nanoparticles in the size range above ~5 nm has been left unattended for a long time.

Objective: The present work is aimed at studying interband transition in citrate capped colloidal gold nanoparticles of size between ~5 nm and several tens of nanometres.

Methods: Turkevich method and modified Brust method were used to prepare citrate capped colloidal gold nanoparticles. Transmission electron microscopy was used to determine their size and shape and their formation was explained with simulated figure obtained by Gnuplot programming. Interband transition was studied with the help of UV-Visible absorption spectroscopy.

Results: Dodecahedral citrate capped colloidal gold nanoparticles of mean diameters 31.5 nm, 12.87 nm and 4.69 nm with LSPR peak positions at 528 nm, 524 nm and 509 nm were prepared. The interband peak of nanoparticles of all three sizes was found to be located at about 260 nm.

Conclusion: Interband transition between Fermi level and 5d bands of the larger density of states in citrate capped dodecahedral colloidal gold nanoparticles of size above ~5nm leads to absorbance peak at ~260 nm, indicating a gap of ~4.77 eV between the Fermi level and closely spaced 5d bands. For smaller nanoparticles, absorption due to interband transition becomes more prominent relative to surface plasmon resonance absorption.

背景：柠檬酸盐封端的十二面体金纳米粒子的光学性质在许多领域中都有广泛的应用。带间跃迁在确定金纳米颗粒的光学行为中具有作用。柠檬酸盐封端的胶体金纳米颗粒在约5 nm以上的大小范围内的带间跃迁已被长时间无人注意。

目的：目前的工作旨在研究柠檬酸盐封端的胶体金纳米颗粒的带间跃迁，其大小介于〜5 nm和几十纳米之间。

方法：采用Turkevich法和改进的Brust法制备柠檬酸盐包覆的胶体金纳米颗粒。用透射电子显微镜确定它们的大小和形状，并用通过Gnuplot编程获得的模拟图解释它们的形成。借助紫外可见吸收光谱法研究了带间跃迁。

结果：制备了平均直径为31.5 nm，12.87 nm和4.69 nm的十二面体柠檬酸盐封端的胶体金纳米颗粒，其LSPR峰位置分别为528 nm，524 nm和509 nm。发现所有三种尺寸的纳米颗粒的带间峰位于约260nm处。

结论：在约5nm以上的柠檬酸盐封端的十二面体胶体金纳米粒子中，费米能级与较大状态密度的5d谱带之间的带间跃迁导致在〜260 nm处出现吸收峰，表明费米能级与紧密状态之间存在〜4.77 eV的间隙。间隔5d波段。对于较小的纳米粒子，由于带间跃迁引起的吸收相对于表面等离振子共振吸收变得更加突出。